Many attempts were undertaken to develop a satisfactory agitating system for the composition. However, unsatisfactory results were attained.
Centrifugal pumps and high speed rotating propellors produced undesirable results. Within a relatively short period of time, for example, within minutes to several days, the use of these mechanisms caused the dispersed resin particles, and pigments when present, to flocculate into larger particles which agglomerated and separated from the composition. In effect, the dispersion was broken down and the composition was destabilized, thereby rendering it ineffective for continued coating operations. Also, coatings formed from compositions which were agitated by these mechanisms contained undesirable swirl patterns and texturing. In addition, air was drawn into the system at the pump seals, causing undesirable foaming.
The use of peristaltic-type pumps and Vanton pumps gave undesirable results also. It was found that the resinous ingredient of the composition deposited on critical parts of the pump and jammed it, thereby rendering it inoperative.
The use of a manifold or similar distributor for circulating the composition to agitate it was found to be unsatisfactory also. Circulation of the bath causes destabilization of the dispersed particles, and the orifices of the manifold became clogged with resin.
During the course of development of the present invention, it was found that the design of an agitating system that would avoid the aforementioned problems, that is, destabilization of the coating composition, undesirable appearance of the coating, foaming, and clogging and jamming of equipment, was complicated by other considerations that had to be taken into account in developing a satisfactory system for certain applications. For example, it was found that the application of a substantially uniformly thick coating to three dimensional articles (parts having significant dimensions in three directions--as opposed to two dimensional parts--for example, flat panels), articles having narrow openings, or articles of complex shape was difficult, if not impossible, to accomplish while simultaneously avoiding the aforementioned problems. It was found also that agitating systems which caused the coating composition to froth or foam were undesirable if the froth or foam was mixed into the composition. It has been observed that this caused the formation of grainy or textured coatings which for certain types of applications are considered to be aesthetically unappealing. In addition, foaming can cause "holidays" in the coating which result in poor corrosion performance. Agitators which created vortices in the composition tended to produce these undesirable results.
It should be appreciated that many of the agitating systems that were investigated, as outlined above, are used most satisfactorily in stirring or agitating conventional coating compositions, for example, latex paint baths (a non-acidified aqueous dispersion of resin particles), electrophoretic paint baths of the type used in electrocoating and comprising water solubilized resins, and inorganic coating solutions, for example, zinc phosphate and chromate coating solutions. However, when used with the more recently developed unique compositions of the type described in the aforementioned patents and in which the organic coating-forming material is present in the acidified composition in the form of dispersed particles, long-term, problem-free agitation of the composition was not attained.
Accordingly, it is an object of this invention to provide for agitation of an acidic aqueous coating composition containing dispersed particles of an organic coating-forming material in a manner such that the stability of the composition is prolonged.
It is another object of this invention to provide for agitation of the aforementioned type of coating composition in a manner such that coatings formed from the composition are smooth and glossy and not grainy or textured.
Another object of this invention is to provide for agitation of the type of coating composition in a manner such that a substantially uniformly thick coating is formed on an article, including articles of complex shape, those having narrow openings, and three-dimensional articles.
An additional object of this invention is the provision of agitating equipment which is capable of continued, problem-free operation when agitating a coating composition of the type referred to hereinabove.